1. Field of Invention
The present invention relates to the light source of a scanner. More particularly, the present invention relates to an installation for improving a scanning range of a scanner along an axial direction of a light source.
2. Description of Related Art
In general, a scanning system can be roughly classified as a reflective type or a light-penetrating type. In the reflective scanning system, a document is placed over a transparent glass panel. Light from a light source travels through the transparent glass panel and impinges upon the document. The light is reflected back from the document to reach an optical sensor after passing through a set of optical elements. The optical sensor for forming a scan image, for example, can be a charge couple device (CCD). In the light-penetrating scanning system, the document is also placed on top of a transparent glass panel. However, light from a light source has to pass through the document, the transparent glass panel and a set of optical elements before arriving at the optical sensor.
Hence, both the reflective and the transparent type of scanning system operate on similar principles. In a reflective scanning system, brightness variation of reflected light from a document is gauged to produce a scan image. On the other hand, in a transparency scanning system, brightness variation of light after passing through the document is gauged to produce a scan image.
However, the conventional linear light source of a scanner has one major drawback. Brightness level close to the mid-section along the axial direction of a light source is higher than the brightness level on each side. Hence, image produced by the light source is brighter in the middle. Since a scanner works according to differences in brightness level, capacity for detecting brightness contrast near both ends of the light axis is lower. If the variation in brightness level along the axial direction of the light source is large, a section close to both ends of the light axis is unsuitable for forming any images.
Several means have been proposed to reduce brightness variation along the axial direction of the light source, several means are proposed. For example, in an invention described in Taiwan Patent No. 244013 titled ‘Improved brightness compensation of an optical scanner through a modified lamp shade structure’, brightness variation along the axial direction of a light source is improved by modifying the lampshade.
In another invention described in Taiwan Patent No. 204363 titled ‘Image-forming optical system for a scanner’, the external structure of the lamp tube is directly modified. For example, two lamp tubes are used instead of one so that compensatory back reflection is increased. Consequently, brightness levels between the central and the side portions along the axial direction of the lamp tubes are almost identical when onto a document. However, the invention demands major modifications of many structural elements. Hence, cost of production is high.
In another invention described in Taiwan Patent No. 352886 titled ‘A lens structure and its integration with an image-reading device’, a method of improving brightness level along the axial direction of a light source is proposed. By changing the degree of reflectivity of a coated film on the reflecting lens inside a scanner, brightness level variation is reduced. However, the reflectivities of more than one reflecting lens need to be modified, thereby increasing the production cost necessary to achieve the results.
In short, the smaller the variation in brightness level along the axial direction of a light source, the better the scanning width of a document and brightness contrast are. Although all the conventional methods are capable of reducing brightness variation along the axial direction of a light source, major structural modification of components are unnecessary.